The pronounced tendency for the queen to expand her brood neat upward is apparently the result of a temperature response. Whenever empty combs are made available directly above the active brood nest, a gradual rise in temperature takes place between these combs surfaces, a condition which is not noticed when combs are made available to either side or below.  The temperature usually reaches 93° within 3 to 5 days and the queen immediately expands her egg-laying to the combs above (except in the case of newly drawn combs). This situation is in harmony with the principle that management applicable to the 2-story brood chamber, namely the periodic reversal of brood chambers. This establishes optimum conditions for the expansion of the brood nest, one of the conditions essential for the elimination of the swarming impulse. 

Empty combs below the brood or above a barrier of several inches of sealed honey are not used effectively for either the expansion of the brood nest or storage of honey. The tendency to expand the brood nest upward insures the brood nest being in close contact with the honey supply and one which is favored by the higher temperatures above. The surplus pollen is placed immediately surrounding the entire surface of the spherical brood nest in a rather uniform layer or band. Surplus honey is placed largely above and to the sides, but never below, for permanent storage. Brood combs placed next to the hive wall were not maintained at full brood-rearing temperature and the development of the unsealed stages was materially retarded. However, the queen in a normal colony provided with two brood chambers avoids the use of outside combs. 

Farrar, C. L. (1931). A measure of some factors affecting the development of the honeybee colony.

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